Polyarylene sulfides as typified by polyphenylene sulfide (hereinafter polyarylene sulfide may be abbreviated as PAS) are engineering plastics having excellent heat resistance, frame retardancy, chemical resistance, electric insulation, moist heat resistance, mechanical strength and dimensional stability. PAS is moldable into a variety of molded products, films and fibers by a variety of molding techniques such as injection molding and extrusion molding and is accordingly practiced in a wide variety of fields including electric and electronic components, machine components and automobile components.
A specific production method of PAS has been proposed to use the reaction of an alkali metal sulfide such as sodium sulfide with a polyhalogenated aromatic compound such as p-dichlorobenzene in an organic amide solvent such as N-methyl-2-pyrrolidone. That method is widely used as the industrial production method of PAS. That production method, however, has some problems, i.e., need for the reaction under the high temperature, high pressure and strongly alkaline conditions, need for an expensive high boiling-point polar solvent such as N-methylpyrrolidone, energy-intensive with high cost for recovery of the solvent and need for enormous processing cost.
Additionally, the polymerization reaction is the desalting polycondensation mechanism and thereby produces a significant amount of byproduct salts such as sodium chloride. After the polymerization reaction, removal of the byproduct salts is needed. The general treatment, however, has difficulty in complete removal of byproduct salts. Commercially available, general-purpose polyphenylene sulfide products contain alkali metal salts as byproduct salts. The weight ratio of the alkali metals in PAS is about 1000 to 3000 ppm. Leaving the alkali metal salts in the polymer product causes a problem such as deterioration of the physical properties such as electrical properties. Such deterioration of electrical properties by the alkali metals contained in the PAS interfered with application of molded products using such PAS as the raw material in the field of electric and electronic components.
The commercially available PAS produced by that method contains 2000 to 4000 ppm chlorine at the terminals. With the object of reducing the environmental load, halogen-free has recently been promoted in especially in the electrical and electronic industries. For example, the guidelines for electric and electronic components, e.g., JPCA (ES-1-2003), IEC (61249-2-21) and IPC (4101B), request that the content of chlorine atom should be reduced to or below 900 ppm. The chlorine content in PAS accordingly interferes with application of PAS to electric and electronic components.
Moreover, PAS obtained by that method is a polymer having a very high polydispersity expressed by the ratio of the weight-average molecular weight to the number-average molecular weight and a very wide molecular weight distribution (Mw/Mn) as 5.0 to 20 and including a significant amount of low molecular-weight components. In application of the above PAS to the molding process, the low molecular-weight components deteriorate the properties such as the mechanical strength and the chemical resistance and disadvantageously interfere with exertion of sufficient mechanical properties. The above PAS also has other problems such as a significant amount of gas components when being heated and a significant amount of eluent components when being exposed to a solvent. To solve such problems, a process of increasing the molecular weight by, for example, oxidative cross-linking under heating in the air has been proposed (for example, JP S45-3368 B). That method, however, causes problems, for example, complicating the process, deteriorating flowability and moldability due to the high molecular weight components produced by oxidative cross-linking under heating, and reducing the productivity.
One proposed method to solve one of the above problems of PAS, i.e., a significant amount of low molecular weight components and a wide molecular weight distribution, causes phase separation of a PAS mixture including impurities into a polymer melt phase including PAS and a solvent phase mainly comprised of a solvent at higher temperatures than the minimum temperature at which PAS is included in the melt phase and thermally extracts the impurities to purify the PAS. Another proposed method deposits and recovers a polymer in granular form by cooling. Those methods extract the impurities by the thermal extraction effect and are thus expected to reduce the metal content in the PAS and narrow the molecular weight distribution. Those methods, however, have only insufficient effects and use expensive organic solvents, which results in the complicated process (for example, JP H01-25493 B and JP H04-55445 B).
As another method has been disclosed a production method of PAS characterized by washing PAS, which is obtained by the reaction of a sulfur source with a dihalogenated aromatic compound in an organic polar solvent, with the organic polar solvent under the temperature condition of 100 to 220° C. The obtained PAS has a molecular weight distribution (Mw/Mn) in the range of 2 to 5. This method, however, has a low yield of PAS and has only the insufficient effect on the molecular weight distribution; the lowest polydispersity of the actually obtained PAS is only Mw/Mn=2.9. Additionally, that method has many other problems to be solved, for example, using a large amount of an expensive lithium compound for polymerization of PAS to have poor economic efficiency and causing some amount of lithium to remain in the PAS (for example, JP H02-182727 A).
As described above, all those methods still have many problems, i.e., poor economic efficiency and low yield resulting from using a large volume of a solvent or needing a complicated process for extraction and purification to obtain a polymer having a sufficiently narrow molecular weight distribution.
A production method of PAS by heating a cyclic PAS has been disclosed as another production method of PAS. (In the description below, a polymer of the higher degree of polymerization obtained by polymerizing polyarylene sulfide as the polymerization material including cyclic PAS and/or linear PAS may also be simply called PAS.) That method is expected to obtain a high molecular-weight PAS having a narrow molecular weight distribution and a less weight reduction by heating. The lower purity of the cyclic PAS, however, tends to produce the lower molecular weight of PAS. It is accordingly preferable to use a highly pure cyclic PAS oligomer that substantially includes no linear PAS as the polymerization material. Only a tiny amount of linear PAS is thus allowed to be mixed in the cyclic PAS oligomer. In general, a cyclic oligomer is obtained as a mixture with a significant amount of a linear oligomer, so that sophisticated purification operation is needed to obtain a highly pure cyclic body. This results in increasing the production cost of PAS, and a more practical method has accordingly been demanded (for example, WO 2007034800).
A polymerization method of polyphenylene sulfide has also been known to heat a mixture of cyclic polyphenylene sulfide and linear polyphenylene sulfide as the polymerization material (Polymer, Vol. 37, No. 14, 1996, pages 3111-3116). That method is a simple polymerization method of polyphenylene sulfide but is not practically applicable, due to its low degree of polymerization of the resulting polyphenylene sulfide. Polymer, Vol. 37, No. 14, 1996, pages 3111-3116 teaches that the higher heating temperature enhances the degree of polymerization. The resulting molecular weight, however, does not yet reach the practically applicable level. Polymer, Vol. 37, No. 14, 1996, pages 3111-3116 also cannot avoid the cross-linked structure and is capable of producing only a polyphenylene sulfide having poor thermal properties. A polymerization method of polyphenylene sulfide having the higher practical applicability and the higher quality has accordingly been demanded.
A known method uses a variety of catalyst components (for example, compounds having radical generating ability or ionic compounds) to accelerate increasing the molecular weight during conversion of cyclic PAS to a polymer of the higher degree of polymerization. More specifically, the disclosed method polymerizes a cyclic arylene sulfide oligomer by ring-opening polymerization under heating in the presence of an ionic ring-opening polymerization catalyst. That method is expected to obtain a PAS having a narrow molecular weight distribution. That method, however, uses an alkali metal sulfur compound, such as sodium salt of thiophenol, as the ring-opening polymerization catalyst for synthesis of PAS and accordingly has a problem that a significant amount of the alkali metal remains in the resulting PAS. An attempt to reduce the remaining amount of the alkali metal in the resulting PAS by using a decreased amount of the ring-opening polymerization catalyst in this method results in another problem, insufficient molecular weight of the resulting PAS. In that method, purification using a solvent is expected to reduce the remaining alkali metal to some extent. Such purification, however, uses a large amount of solvent in the production process and has disadvantages such as poor economic efficiency and low yield. The polymer obtained by that method has a polymerization initiator component remaining in one of the terminals, which causes decomposition under heating and leads to the unsatisfactory level of gas generation amount (for example, JP H05-301962 A, JP H05-163349 A and JP H05-105757 A).
A method disclosed to solve the problem of the PAS obtained by the above method, i.e., to reduce the remaining amount of the alkali metal in the resulting PAS, is a production method of PAS that polymerizes a cyclic aromatic thioether oligomer by ring-opening polymerization in the presence of a polymerization initiator that produces sulfur radical by heating. That method uses a non-ionic compound as the polymerization initiator and is thus expected to reduce the content of the alkali metal in the resulting PAS. The polyphenylene sulfide obtained by that method, however, has a low glass transition temperature as 85° C. This is because the resulting polyphenylene sulfide has a low molecular weight and includes a significant amount of low molecular-weight components to have a wide molecular weight distribution. That method accordingly still has the problems of the molecular weight and the molecular weight distribution. There is no disclosure on the weight reduction ratio by heating the polyphenylene sulfide obtained by this method. The polymerization initiator used in that method has a lower molecular weight and poorer thermal stability than polyphenylene sulfide. There is accordingly a possibility that the polyphenylene sulfide obtained by this method generates a large amount of gas by heating and has poor molding processability (for example, U.S. Pat. No. 5,869,599).
Those methods of producing cyclic PAS generally produce cyclic PAS in the powdery form (for example, WO '800, JP 2009-030012 A and JP 2009-149863 A). We found a characteristic problem that the powdery cyclic PAS mixture is compacted in a screw feeder and fails to be conveyed in the course of feeding to an extruder using the screw feeder. More specifically, the powdery cyclic PAS mixture has poor conveyance, which causes the phenomenon that the powdery cyclic PAS mixture gradually accumulates in the screw feeder and is eventually compacted not to be conveyed. This interferes with taking advantage of the inherent characteristics of the cyclic PAS, i.e., low gas generation and improved melt processability and using the powdery cyclic PAS mixture by an industrially simple method. The cause of such compaction is not clear, but it is presumed that high affinity between rings and high cohesiveness of powder of the cyclic PAS which is a cyclic oligomer may cause poor conveyance.
A production method of cyclic PAS by melting cyclic PAS by heating to an amorphous form has been disclosed as a method of recovery of cyclic PAS other than the powdery recovery method having the above problems. That method dissolves a cyclic PAS having high crystallinity and low solubility to convert the cyclic PAS into an amorphous form and cools down and solidifies the cyclic PAS in the amorphous form for recovery. That method is expected to enhance the solubility of the cyclic PAS and thereby improve the ease of handling. With respect to the cyclic PAS obtained by that method, however, there is no disclosure on the gas generation amount which is important in melt processing or on the weight reduction ratio. There is also no description on the ease of handling during the molding process other than the solubility or on pelletization. The description only regards the solubility of cyclic PAS (for example, JP 2010-018733 A).
It could therefore be helpful to provide a production method of a polyarylene sulfide having a narrow molecular weight distribution, low gas generation and high industrial usability without employing a process of increasing the purity by purification of a cyclic polyarylene sulfide using a solvent or a complicated process of increasing the molecular weight such as oxidative crosslinking by heating of a polyarylene sulfide after polymerization, as well as to provide a cyclic polyarylene sulfide pellet having excellent handling characteristics such as ease of conveyance and high molding processability, low gas generation and high industrial usability and a production method of such pellet.